1. Field of the Invention
The present invention is related to an apparatus which receives control information sent from a transmitter, and which is ideally suited for use as a remote controller in applications such as the so called keyless entry system for automobiles.
2. Description of the Prior Art
The keyless entry system is an apparatus which can control the locking and unlocking of an automobile's doors by remote control, even from a location at some distance from the automobile. This apparatus makes it possible for someone such as the driver to carry the transmitter, and without using a key, to lock or unlock the doors and trunk etc. by remote control, through an operation such as the pressing of a push button switch on the transmitter.
In the keyless entry system, a specific set of identification data is established in advance between a transmitter and receiver which form a set. When the aforesaid push button switch is pressed, this identification data undergoes frequency modulation and is transmitted.
When the received electric field strength, detected by a squelch circuit in a standby mode, exceeds a preset threshold value, the receiver takes up the data and performs collation of the above mentioned identification data or the like. In this way, when the identification data match, the previously mentioned operations such as locking or unlocking of the doors are performed.
Thus, the receiver will produce a received response and perform door locking/unlocking control, only with respect to a control signal from a transmitter which matches the identification data registered in the receiver. Based upon this, improvements have been devised for security functions such as theft prevention.
On the one hand, theft prevention apparatuses have been extensively developed in recent years. This is the type of apparatus which detects the entry of a thief into the car without the use of a proper key, and which then generates an alarm. The current type of theft prevention apparatuses are constituted so that, for example, they detect the breaking of a window and then generate an alarm. However, a problem still remains that even if the car's owner discovers from a distance that a thief is prowling around and is about to break into the car, he is not able to sound the alarm and prevent damage to the car before it occurs.
Accordingly, there has been a desire for a theft prevention apparatus with further improved security functions, through a combination of the keyless entry system's transmitter and receiver with the current type of theft prevention apparatus. However, the addition of these extra functions to the transmitter and receiver would complicate their construction.
Furthermore, the previously mentioned identification data is, for example, stored in a ROM (Read Only Memory), and the transmitter and receiver are equipped with these ROM's. Therefore, when the transmitter is lost, it is necessary to exchange the ROM in the receiver with one that matches the identification data of a new transmitter, and maintenance operations become very troublesome.
Still further, in order to avoid malfunctions at places such as parking lots, a large number of identification data combinations are created. Because of this, there is also the problem of mounting costs for the management of maintenance parts, since it is necessary for the manufacturer to maintain a stock of ROM's which correspond to all of the identification data combinations.